Hollow core photonic band gap (HC-PBG) fibers have been fabricated from silica glass and reported in the literature. See, e.g., Cregan et al., “Single-mode photonic bad gap guidance of light in air,” Science, 285(5433), 1537-1539 (1999); Barkou et al., “Silica-air photonic crystal fiber design that permits waveguiding by a true photonic bandgap effect.” Optics Letters, 24(1), 46-48 (1999); and Venkataraman et al., “Low loss (13 dB/km) air core photonic band-gap fibre,” ECOC, Postdeadline Paper PD1. 1 Sep. 2002, all of which are incorporated herein by reference. FIG. 1 shows a schematic of the cross-section of a HC-PBG fiber. The periodic layered structure of air holes 100 and glass 110 creates a photonic band gap that prevents light from propagating in the structured region (analogous to a 2D grating) and so light is confined to the hollow core. Typically, the periodicity of the holes is on the scale of the wavelength of light and the outer glass is used for providing mechanical integrity to the fiber. The fact that light travels in the hollow core also means that the losses will be lower so longer path lengths can be used. Also, non-linear effects will be negligible and damage thresholds will be higher so that higher power laser energy can be transmitted through the fiber for military and commercial applications. Also, since light is guided in the hollow core, an analyte disposed therein will have maximum interaction with light, unlike traditional evanescent sensors.
The periodicity of the holes, the air fill fraction (defined by the ratio of void volume to solid material volume in the microstructured region, i.e., the region comprising the plurality of holes and solid material therebetween, and exclusive of the core and jacket regions), and the refractive index of the glass dictate the position of the photonic band gap, namely the transmission wavelengths confined to the hollow core and guided within the fiber. HC-PBG fibers are obtained by first making a structured fiber preform and then drawing this into a microstructured fiber with the correct overall dimensions. The fiber preform is typically comprised of a central structured region, which can be made, for example, by stacking tubes, extrusion or templating, which is inserted into a supportive outer jacket tube. This assembly process inevitably introduces voids between the central region and the outer jacket tube. These voids can be similarly sized to the intended holes in the structured region of the fiber preform, or even larger, and run the entire length of the fiber preform, therefore making fiberization difficult. This is especially true for specialty oxide and non-oxide glasses where the vapor pressure during fiberization may be sufficient to prevent collapse of these interstitial voids.
In the fabrication of silica glass microstructured fibers, there is at least one method where the softening point temperature of the inner structured region is higher than that of the outer jacket by at least 50° C. but no more than 150° C., such that during fiberization the structured region remains relatively firm and is less susceptible to deformation (U.S. Pat. No. 6,847,771 to Fajardo et al., the entire contents of which is incorporated herein by reference). However, this method does not work for non silica specialty glasses, especially non-oxides and chalcogenides, due to their low softening temperatures and higher vapor pressures.
There are no HC-PBG fibers reported using specialty glasses. This is partly due to the fact that high air fractions are needed. Specialty glasses tend to be more fragile and, therefore, difficult to make and handle the microstructured fiber preforms.